Optical disk recording media which are now put to practical use are CD (compact disk), MO (magneto-optical disk), CD-R (recordable compact disk), CD-RW (rewritable compact disk), etc. In these recording media, a recording film and a reflective film are formed on a polycarbonate substrate of 1.2 mm thick, and a protective layer comprising an ultraviolet-curable coating agent is provided for the purpose of protecting the above films from external causes. Recently, for further improvement of storage capacity, a recording medium has been developed which comprises two laminated substrates with the thickness of the polycarbonate being 0.6 mm which is half the thickness in conventional recording media. By employing such a construction, there are practically used DVD-R, DVD-RW, DVD-RAM, DVD+R, DVD+RW, etc. which have solved the problems of birefringence of polycarbonate substrates and reduction of laser spot diameter. In all of these recording media, a recording film, a reflective film, and the like are formed on a polycarbonate substrate of 0.6 mm, and an ultraviolet-curable protective layer or adhesive layer is provided for the purpose of protection or adhesion as mentioned above.
However, DVD recording media are still insufficient in capacity as recording media for attaining large capacity in the times of digital broadcasting. Thus, optical disks comprising a substrate on which a recording layer and a transparent layer of 100 μm are laminated are proposed and practically used as the next-generation high-density optical disks (Patent Document 1). In these optical disks, writing and reading are carried out with blue laser beams from the transparent cover layer side, and not from the polycarbonate substrate side.
The methods for forming the cover layer includes a method of laminating a transparent layer of 100 μm and a method of forming a layer of 100 μm on the recording layer using an ultraviolet-curable resin. For the formation of the ultraviolet-curable resin layer, 2P method and a spin coating method are proposed. As the ultraviolet-curable resin formed on the recording film, there are proposed, for example, 2P agents such as compositions disclosed in Patent Document 2 and Patent Document 3. Furthermore, there are proposed protective coating agents such as compositions disclosed in Patent Document 4, Patent Document 5 and Patent Document 6.
Patent Document 1: JP-A-11-273147
Patent Document 2: JP-A-5-059139
Patent Document 3: JP-A-5-132534
Patent Document 4: JP-A-3-131605
Patent Document 5: JP-A-3-172358
Patent Document 6: JP-A-2003-268263